Observations of tidally-varying ocean bottom pressure near the grounding zone of the Ross Ice Shelf

Ice-sheet stability is sensitive to conditions at ice-shelf grounding zones, the transition between grounded and floating ice, at which buttressing forces and basal melt rates are generally higher than elsewhere on the ice shelf. Grounding zone dynamics are complicated by ice-shelf flexure within the first 5-10 km downstream of the floatation limit, a nonhydrostatic flexure zone. We present the first oceanographic observations of nonhydrostatic tidal dynamics in a flexure zone from pressure sensors installed in the sub-ice-shelf cavity of the Ross Ice Shelf in 2015. Tidal pressure fluctuations in the flexure zone are found to be of the same magnitude as hydrostatic tidal pressure fluctuations immediately downstream of the flexure zone and significantly greater than the hydrostatic pressure variations at the site, inferred from surface GPS elevation changes. Thus, the sub-ice-shelf ocean cavity in the flexure zone is over-pressurized at high tide and under-pressurized at low tide. This tidally-varying ocean pressure at the grounding zone (on the order of 1 dbar peak-to-peak) may modify subglacial hydrologic flow where there is connectivity between the subglacial hydrologic system and the sub-ice-shelf ocean cavity. Furthermore, we detect phase lags between the tidal pressure signal in the ocean cavity and the ice-shelf elevation response on the order of 10s of minutes, providing insight into the ice shelf's rheology. These observations lay the foundation for representations of the flexure zone and its nonhydrostatic tidal dynamics in ocean circulation models of sub-ice-shelf cavities. Future work is needed to determine whether these model advances are needed to accurately reproduce ocean circulation and ice-shelf melt rates near grounding zones.